You have explained it all very well. Raghu really does need to decide what he wants and make a strong decision.
If he wants to be with Sumi and make a commitment to her than break his relationship with Shivani, he can still watch over Shivani without continuing with the marriage.
If he wants to be with Shivani then he needs to stop giving Sumi the hope that they will be together and tell her to move on.
Unfortunately deep down he really doesn't know what he wants and doesn't want to hurt either but eventually one of them will be hurt which will be Sumi and who can blame her for what she may do.